AbstractIntroductionThe gestation when small for gestational age (SGA) is first associated with asthma is not well understood. Here, we use routinely acquired data from 10 weeks gestation to up to 28 years of age to test the hypothesis that SGA before birth is associated with an increased risk for asthma in a large population born between 1987 and 2015.MethodsDatabases were linked to produce a single database that held antenatal fetal ultrasound measurements; maternal characteristics; birth measurements; childhood anthropometric measurements at age 5 years; hospital admission data (1987–2015); and family doctor prescribing (2009–2015). Asthma admission and receipt of any asthma medications were the outcomes. Analyses related single and then multiple anthropometric measurements to asthma outcomes.ResultsOutcome data were available for 63,930 individuals. Increased length in the first‐trimester size was associated with a reduced odds ratio (OR) for asthma admission of 0.991 [0.983, 0.998] per mm increase and also a shorter time to first admission, with a hazard ratio risk of 0.987 [0.980, 0.994] per mm increase. Independent of all earlier measurements, increased height at 5 years (available in a subset of 15,760) was associated with reduced OR for an asthma admission, with OR of 0.874 [0.790, 0.967] per z score. Longitudinal measurements of weight were not related to asthma outcomes.ConclusionsLonger first‐trimester length is associated with more favorable asthma outcomes, and subsequently, increased height in childhood is also independently associated with more favorable asthma outcomes. Interventions that reduce SGA and encourage healthy postnatal growth might improve asthma outcomes.